Identical Twins: Why does the fertilized egg split?

The human body performs some incredible works of magic, and identical twins conception is one of them. Why does a fertilized egg split to create identical twins? It is well established that embryo splitting is not a hereditary tendency and yet around 0.3% of babies are born a natural clone. So what causes identical twins?

The quick answer is… no-one knows. But like all wonderous outstanding questions, there are plenty of scientists trying to figure it out. These guys are busy looking at when the egg splits to form twins, environmental factors and unusual occurrences of repeat identical twins… are we really sure it doesn’t run in the family?

Before we consider the scientific theories below, here’s a summary of what we do know:

How identical twins form

Unlike fraternal twins (which come from the fertilization of two eggs), identical twins are formed when a single zygote (fertilized egg) splits into two. This natural cloning process will happen at some point in the first ten days of gestation, but the exact time the embryos split varies, creating different ‘types’ of identical twins.

The different types of identical twins:

If the split happens early in day 1 after fertilization the twins will form separate placentas and amniotic sacs, these are medically called Dichorionic/Diamniotic (di/di) twins.

It is more likely the split with occur around days 4-5. By this time the twins will have time to form separate sacs but will end up sharing a placenta. These, the most common type of identical twins are called Monochorionic-Diamniotic (mono/di) twins.

1% of the time the split happens around day 9-10 meaning the babies end up sharing a sac. Monochorionic-Monoamniotic (mono/mono or mo/mo) twins are sometimes ‘mirror twins’, causing quirks such as first teeth cropping up on opposite sides of the mouth, and in some cases, even internal organ positions are mirrored.

Very rarely, the embryo splits around day 13-15, making it impossible for the twins to separate fully, causing the babies to be born conjoined.

The embryo splitting process:

After a human egg is fertilized, the zygote ‘collapses’ and expands a number of times as the cells multiply. It’s during one of these ‘collapses’ that the magic moment happens. During a collapse, the cells contained in the embryo divide into two groups, allowing two separate embryos to ‘hatch’ from the protective outer layer of what is now called the blastocyst. These two matching balls of cells keep dividing and multiplying to create two separate, yet identical, babies.

‘Twin Towns’

Allahabad in North-East India is just one of a number of ‘twin towns’. For the past 40years, Allahabad has experienced an unusually high number of identical twin births. Locals are sure there is something in the water or soil causing this, as they claim animals such as buffalo are also affected by the phenomenon. However, scientists are yet to find any environmental or social oddities and DNA from Allahabad twins has not given much insight either. Most researchers have written off the theory of twin towns as no more than a statistical fluke. Another twin town, Linha São Pedro in Brazil, reported in the 1990’s that 5% of births were identical twins… that’s quite the fluke.

The Nine-Banded Armadillo

Nine-banded armadillos almost always give birth to four identical quadruplets. These quadruplets are created from one single fertilized egg that splits and then splits again. This is thought to be an evolutionary result of the physical constraints imposed by the shape of the armadillo’s uterus. However, while it may be seen as an evolutionary advantage for the Armadillo, twinning of embryos in humans is considered more of a pregnancy fail by scientists, because it holds higher risks of congenital anomalies and premature birth. Still, for armadillo, it’s not a random event so something is causing it to happen…

Identical twins and IVF

When receiving IVF treatment, the chances of having identical twins rises from 0.3% to 2%. So even if only one embryo is implanted there is a 2% chance it will divide and create two babies. No-one knows why the chances of embryo division is higher, although it is suggested that subtle chemical differences between the lab and the human body are to blame. Maybe it’s just all that prodding and poking?

SCIENTIFIC THEORIES AS TO Why does the fertilized egg splitS…

We’ve looked at when it happens, but what scientific theories are out there for why identical twins develop from one fertilised egg?

Genetic mutation:

Dr Bruno Reversade has been busy investigating variations in the genomes of families from ’twin towns’ in the hope of finding a twinning gene. He has a candidate region, on chromosome four, and he thinks mutations in a gene here might have been present in the founders of twin towns, and then spread through the population. He speculates that the mutated gene might prevent cells sticking together tightly within the blastocyst, resulting in a split.

Dr Dianna Payne thinks the imperfect environment of the IVF lab is causing some cells to die or weaken at the cell junctions. This leads to the idea that a similar process could be caused in naturally conceived embryos, if they were triggered by faulty genes. It is also possible that subtle differences in the cells could force cells to repel one another, pushing two separated groups of cells to opposite sides of the blastocyst.

An enzyme in sperm:

There is a popular theory (although I have struggled to find any scientific sources) that an enzyme in sperm causes the embryo to split. Many families with twins claim they have an abnormal number of identical twins in their family tree, even though at present science still claims it’s a random event. Twin dads like to consider themselves carriers of super sperm… maybe they’re right?

It’s all about timing:

Dr Judith Hall suggests twinning depends on the timing of fertilization, explaining why humans twin more than other animals (except the armadillo of course!). Most mammals choose to mate when conditions are perfect, when eggs have been freshly ovulated. Humans on the other hand just do it any old time and an old egg may be more likely to split.

So…there you have it.

I think if you combine Payne and Hall’s ideas to suggest an old egg is ‘faulty’, leading to abnormalities which create a repulsion between the cells, you have a fairly sound theory. After all, most identical twins don’t make it – it has been suggested that 12% of natural conceptions produce identical twins – but the vast majority of embryos are lost. Occasionally this can even result in a ‘vanishing twin’ where one embryo continues to full term while the other is lost and absorbed by the body.

Essentially something has gone wrong with the usual fertilization process to create two babies from one, although as an identical twin mom myself I think it’s just magic. Take your pick, what do you think causes identical twins?

I vote for magic, or miracle, or mystery. I’m like Heather, mysteries yet to be solved! That keeps us humble and identical twins a marvel.
I really enjoyed learning from your essay. I like the way you gave titles and examples. So very interesting!

An interesting read. In my family, every alternated generation has a twin. SO it looks like its sort of genetic. My granfathers grandfather had a twin. It then moved to my grand father who himself was a twin. My sister then delivered twins.

Interesting! These were all identical twins? Fraternal (non-identical) twins are indeed hereditary as its the tendency to release multiple eggs that is passed on in the genes. I always think its interesting how things skip generations, I’m sure there’s a reason… maybe I’ll look it up for another post!

Fraternal (non-identical) twins do indeed run in families. Some can look really similar, others really different, just like regular brothers and sisters. It’s all so fascinating indeed! Thanks for reading 🙂

Wow! This is absolutely fascinating!! I, like a lot of others, find identical twins soooo interesting! When I was little, I always wished so much that I had a twin! Ive never actually looked into why this happens though-the possible explanations outlined here would be so interesting to explore further. I didn’t realise that it’s seen as kind of a body ‘fail,’ and that 12% of conceptions are twins, but so many don’t make it. The old egg theory does seem the most likely. This is all so interesting! My cousin married into a family where there’s so many twins-but the non identical variety, which you’ve said aren’t really mysterious. But her husband is a twin, he also twin sisters, his dad was a twin, and his brother and sisters have all had twins!! It blows my mind a little!!
#bigpinklink

Thankyou. And it’s not crazy to want twins at all… they are awesome! Good luck with your quest… maybe there are some good old wives’ tales for weird stuff you can do to improve your chances?! Haha. I will have to look that up for a future blog post ?

TwinPickle, i love your articles! My best friends all through school were (well, still are!) identical twins. absolute mirror image until about 8th grade, and starting with one or two features changing! Genetics behind twins are so fascinating. thanks for another great article!!

Thanks for your enthusiasm, I’m so glad you find it as fascinating as I do! One of the boys managed to chop the end off his finger a couple of weeks ago so we didn’t quite make it to 8th grade remaining mirror image! ??

My husband’s family is certainly an anomaly. His grandparents had 3 sets of identical twins. Every brother in the grandfather’s family had a set of identical twins. My husband and I have Didi identical twins. They are the 15th set of identical twins in the family.

Hey!
I was wondering if you knew where I could find info on the chances of me having twins. My mother is a twin and my fathers-father was a twin so I believe I get the gene from both parents. What is the likely hood I will have twins?

Hi Anna! I’m not sure how to calculate your chances… it really depends what you believe in terms of the theories. Fraternal twins run down the Mother’s side, because it’s the tendency to release multiple eggs that runs in the family. So if you’re fathers-father was a fraternal twin that will not affect you. However… if he was an identical twin and you believe in the sperm enzyme theory (which many do), your chances are increased!

I got to tour article from a google-search. I have identical twins and a «singleton». Would have loved to have twins at every birth. They have so much joy from eachother! Wish I knew what caused the split so that I could have more sets <3

I am an identical twin. I am 78 years old. My twin died when we were 50. We thoroughly enjoyed being twins, and we often spoke of twinship in philosophical terms. The question of “Why identical twins?” and “Why does the zygote split” fascinate me. One thing I resent is the characterization of twinning as an accident, a failure, something dangerous to be avoided. What if we think of twinship in terms of teleology; that is, in terms of what is the purpose of identical twins? Some of the theories my twin and I came up with are: that an extraordinary friendship in one life comes to fruition as identical twins in a future life; that identical twins teach the world of singleton just what the perfect human community looks like; that out of difficult and dangerous developments can come events which completely go against what is considered normal. My twin and I even imagined a Twin Liberation Movement. The problem? Too few identical twins to make an effective movement.

Hi George (one of my twins boys is also called George!!)… It’s great when grown up twins drop by at TwinPickle to share stories! I agree that science writing twinning off as a failure is awful, it really is magical! I love the idea of a friendship reincarnating as twin siblings, such a heart-melting thought. And I totally agree that we can learn so much about community and relationships from watching the built-in nature of twins love for each other. When you start the Twin Liberation Movement let me know, haha! I would love to know the name of your brother… it wasn’t Arthur by any chance was it?! You must miss him dearly, I can see in my boys that no outside relationship will ever be quite the same as theirs.

Thank you, Katherine, for your thoughtful and caring response to my post. You have an appreciation and understanding of twins that is quite wonderful. My twin’s name was James, but I always called him Jimmy and he always called me Georgie, the names we used in our childhood.
If you and your readers would like to know more about us, please read the cover story on us (“Mystical Bond”) in the magazine section of the Washington Post, July 16, 1988, a copy of which is in my blog “Twins in a World of Singletons” at https://rosstwins.blogspot.com/

I am expecting identical twins grandbabies which are due the end of July but hopefully not till at least June. They are mo/di twins. At first they thought they were mo/mo twins because they couldn’t see the membrane showing 2 sacs. They have twin to twin transfusion which is extremely scary. At 16 weeks my daughter in law had surgery inside her womb to separate blood vessels and split the placenta into 2. She’s on semi bed rest and goes every week to the specialist for 2 hours of sonograms to make sure both babies hearts are good and they have an ample amount of amniotic fluid, both drinking and peeing… It’s been 2-1/2 weeks and they are doing great. They are both 9oz and they are saying the odds are good that she’ll carry them to 32/34 weeks. At first they didn’t think they would make it, but other then mom having a hole in the sac all 3 are doing great. She is cautious because she can leak fluid or the sac could break so she and my son are still very scared and hopeful at the same time. It’s a very exciting time. I can’t wait to meet them. Thank you for writing all your twin articles. Between reading your stuff and doing my research I’ve learned so much..

Hi Lisa! Thankyou for sharing your family’s story. My twins were also mo/Di and I was monitored closely for twin to twin transfusion but luckily for us our boys stayed pretty equal all the way. I hope your littles can stay tucked in Mom as long as possible. I delivered our boys at 36wks but I know many twin moms that have successfully delivered earlier. My thoughts are with you and those babies! ❤️

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